Wire sealers are commonly employed to simultaneously cut and seal plastics films in packaging. In one widely employed system, a center folded film uses its fold line as one side edge seal. The film, with the material being packaged positioned therein, is sealed along its other side edge and back edge simultaneously by use of an L-sealer, having two wires oriented 90.degree. to one another. The seal forming the back edge of the package, when cut, also forms the front edge seal for the next succeeding package.
The sealing mechanism for each arm of an L-sealer comprises a wire or ribbon of a high resistivity metal, such as Nichrome.RTM., Chromel.RTM., Tophet.RTM. and the like, and a back-up pad against which the film and wire are held. Sealing occurs as a pulse of electricity is sent through the wire when held against the back-up pad.
Since most packages are not square, L-sealers typically have wires of unequal length. For example, one commonly employed sealer, a model S-4C, manufactured by Shanklin Corporation, has arms of 35.375 inches (89.853 centimeters) and 25.125 inches (63.818 centimeters). If the same voltage were applied to these unequal length wires, the shorter wire would heat up much faster and attain a much higher temperature than the longer wire, due to its lesser total resistance. By the time the longer wire had reached an acceptable sealing temperature, the shorter wire would be at an excessively high temperature resulting in one or more of a scorched back-up pad, vaporization of the plastics film and resulting smoke and build-up on the back-up pad and wire and/or reduced wire life.
Two methods are currently employed to compensate for the difference in length of the two wires in an L-sealer. In one method, the voltage supplied to each wire is essentially proportional to its length. To accomplish this result, the wires are connected to separate windings of a multiwound transformer, with each winding of the transformer being approximately proportional to the length of the wire it supplies.
In another method, a single voltage supply is used, and the wires are connected in series to the voltage source. This results in an identical current flow through the two wires.
If ideal conditions were obtainable, either of these two methods would result in the wires heating at similar rates and reaching similar temperatures. However, ideal conditions are not real conditions. In actual practice, the two windings of the multiwound transformer may not exactly match the wire length proportion. The heat sink characteristics of the back-up pads for the two wires may not be equal. The two wires may have slightly different diameters, metallurgical state or other condition causing their resistance per unit length to be different. If one or more of these common conditions are present, loss of precision heating control results.
On the other hand, there are times when it is desirable to purposely slightly unbalance the behavior of the two wires in a controlled manner. For example, when sealing thick packages, wrinkles along the back seal might require additional heat to produce a satisfactory seal. Or, if the film has unequal shrink properties in its machine and cross directions, different sealing temperatures may be desirable.
It is desirable, therefore, to more accurately and individually control the supply of electrical power to the wires, enabling compensation for uneven conditions and permitting precise control of desired unbalance.